I learn by "concrete proof in the pudding"...there is none for the FZ200 being diffraction limited at f/2.8...

But if it was (for example in the center of the image), then that would actually be a good thing! If a lens is "diffraction limited" (has its peak resolution) wide open, then that just means that it's a very good lens without any significant aberrations, where the resolution mostly will be limited by the unavoidable diffraction.